Discharge device



Filed April 20, 1962 o o o 3 2 I ANODE-CATHODE VOLTAGE D. C. V.

INVENTOR James E. Peterson BY AfTORNE WITNESSES United States Patent 3,183,393 DISCHARGE DEVICE James E. Paterson, Bethel Park, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pin, a corporation of Pennsylvania Filed Apr. 20, 1962, Ser. No. 189,088 3 Claims. (Cl. 313218) The present invention relates to discharge devices and more particularly to a multi-element hollow cathode device for use in atomic absorption spectroscopy equipment.

The use of hollow-cathode devices as light sources in spectroscopic investigations has long been known and devices of this nature have been widely used in applications requiring sharp spectral lines. Prior art devices, however, have largely been limited to the single element type as it was felt that the use of a multielement cathode would result in a preferential sputtering or boiling out of the lower melting point element to provide a coating of that element on the surface portions of the cathode. This would, in turn, result in the emission of spectral lines of only that one particular element.

It is, therefore, an object of this invention to provide an improved discharge device.

Another object is to provide an improved hollow cathode device for use in atomic spectroscopy.

A further object is to provide an improved multia element cathode.

A still further object is to provide a light source for spectroscopic use which produces light characteristic of a plurality of elements.

Basically the present invention provides a gaseous discharge device for the production of spectral lines of a plurality of elements in which the cathode is comprised of an alloy including calcium, magnesium, and aluminum.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to the accompanying drawings in which:

FIGURE 1 shows a perspective view, partially broken away and partially in section, of a discharge device embodying the present invention; and,

FIG. 2 shows graphically the typical operating characteristic of the device of FIG. 1.

With respect now to FIG. 1 there is shown a discharge device according to the present invention. The device includes an envelope 10, of a suitable material such as glass, having an enlarged tubular portion 12 and a smaller tubular portion 14. The tubular portion 14 is sealed off at one end by a window 16 which is of a suitable material efliciently transmissive of light wavelengths of about 2850 and 4227 angstrom units, for example quartz. The portion 12 is sealed ofi by a button stem header 18, having a suitable tipped off exhaust tube 20, in a manner well known in the art.

Supported by the header 18 and disposed within the portion 12, there is positioned a cathode 22 which, in accordance with the present invention is comprised of an alloy of calcium, magnesium, and aluminum. In the preferred embodiment of the invention, the alloy is comprised of the following percentages by weight: calcium to magnesiuml0 to 15 and aluminum-70 to 80%. This alloy composition range has proven to be one which is highly satisfactory. When the percentage of the calcium and magnesium falls below the 10% level, it has been found that although the spectral lines for these elements are present, the intensity of the lines is relatively weak. It has also been found that when the percentages of calcium and magnesium exceed the upper level of 15 the resulting alloy lacks good mechanical properties. That is, that when these elements exceed the 15% level by any appreciable degree the resulting material becomes mechanically weak with the result that the material has a tendency to crumble and is not readily machinable.

One method by which the cathode alloy may be produced is as follows. The calcium, magnesium and aluminum, perferably in commercially pure form, are placed in a suitable crucible, for example graphite, and the elements are then induction melted in a helium atmosphere at approximately three-quarters atmospheric pressure. The materials are held molten for approximately one minute to permit sutficient time for the alloying to occur and are then bottom poured into a split copper mold to form a cylindrical member of the desired size.

The cylindrical member as was formed above may then be machined to form centrally located here or hollow portion 24 along the axis of the cylinder. The cathode 22 is then aflixed, in electrical contact, to a suitable support 26, which is of a conducting material such as nickel, and positioned within the portion 12. An electrical potential is supplied to the cathode 22 by means of the support 26 which extends through the button stem header 18 in a manner well know in the art.

An anode 28, which is shown to be in the form of a ring, is positioned in close proximity to the cathode near its upper edge 23. The anode is of a suitable electrically conductive material such as tantalum, nickel, or tungsten and is supported from header 1% by means of two support rods 3e. At least one or" the support rods 30 is of an electrically conductive material, for example the same material as that of the anode. The anode is supplied with a suitable source of potential through the support 3i? which extends through the header 18.

After the above assembly has been made, the device is evacuated, filled with an inert atmosphere, for exargon or neon, and sealed oif by means of the exhaust tube 20 in a manner well known in the art of discharge devices. The pressure of the fill is preferably in the range of from 0.5 to 10 millimeters of mercury.

In operation, a direct current potential of appropriate value is applied between the cathode 22 and anode 28 to provide that the cathode 22 is negative with respect to the anode 28. This direct current potential causes the gas within the envelope to ionize and hence establish a discharge between these two electrodes. The discharge is largely concentrated between the interior surface of the bore 24 and the anode 28 and the bombardment of this bore surface by the ions and electrons causes the cathode to emit radiation in accordance with the material of which the cathode 22 is composed, in the present device, calcium, magnesium, and aluminum. This emitted radiation is transmitted through the length of the envelope 10 and passes out of the window 16 where it may be utilized by methods well known in the art to analyze or inspect the desired sample material.

In one specific example of a device built in accordance with the present invention, the envelope 10 was made with an overall length of approximately 6.38 inches and a distance of about 4.5 inches between the top edge of the anode 28 and the window 16. The cathode of this particular device had an outside diameter of about /2 inch, an inside diameter of about inch, an overall length of about %inch and a bore 25 approximately inch in depth. The ring-shaped anode 28 was of titanium and was spaced approximately inch in the radial direction from the outside edge of the cathode.

This device was filled with argon at a pressure at approximately 3 millimeters of mercury. The typical operating curve for this device is shown in FIG. 2. When the above-described structure was operated at a voltage corresponding to an operating current of approximately milliamperes, the device was found to emit radiation having avery sharp spectral peak at a Wavelength of approximately 2852 angstrom units which corresponds to the magnesium and with a second very sharp peak at a wavelength of 4227 angstrom units which corresponds to the calcium.

As is shown, the total volume of the envelope 10 is large compared with the volume occupied by the cathode 22 and anode 28. This large ratio of volumes is for the purpose of including a relatively large amount of gas as it has been found that in operation, the metal vapor emanating from the cathode 22 tends to deposit on the walls of the envelope 10. In so doing, it entraps some of the gas fill. Thus, by providing a large volume of gas, the life of the device is appreciably increased.

It is thus seen that by the present invention there is provided a hollow cathode-type discharge device in which a single cathode provides the spectral lines for more than one particular element.

While there has been shown and described what is at present considered to be the preferred embodiment of the invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangement shown and described and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

I claim as my invention:

1. A light source of the hollow cathode type comprising an anode and a cathode disposed within a gaseous atmosphere, said cathode comprising an alloy of calcium, magnesium and aluminum, said alloy including calcuim and magnesium each in the range of about 10 to 15% by weight and aluminum in the range of about to by weight of the total mass of the alloy.

2. A light source of the hollow cathode type comprising an enveiope, an inert atmosphere within said envelope, an anode and a cathode comprising substantially an alloy of calcium, magnesium and aluminum wherein the proportions of calcium and magnesium each do not exceed 15% of the total mass of the alloy.

3. An arc discharge device comprising an envelope, a gaseous atmosphere within said envelope comprising argon at a pressure of about 3 millimeters of mercury, an anode and a cathode consisting substantially of an alloy of calcium, magnesium and aluminum wherein the calcium and magnesium each constitute about 10 to 15% of the total mass of the alloy.

References Cited by the Examiner UNITED STATES PATENTS 1,334,150 3/20 Green et a1. 313-178 1,834,251 12/31 Moore 313 1,908,366 5/33 Kingsbury 313-218 XR 2,765,417 10/56 Foulke 313-185 XR 3,089,054 5/63 Walsh et a1. 3132l8 DAVID J. GALVIN, Primary Examiner. 

3. AN ARC DISCHARGE DEVICE COMPRISING AN ENVELOPE, A GASEOUS ATMOSPHERE WITHIN SAID ENVELOPE COMPRISING ARGONAT A PRESSURE OF ABOUT 3 MILLIMETERS OF MERCURY, AN ANODE AND A CATHODE CONSISTING SUBSTANTIALLY OF AN ALLOY OF CALCIUM, MAGNESIUM AND ALUMINUM WHEREIN THE CALCIUM AND MAGNESIUM EACH CONSTITUTE ABOUT 10 TO 15% OF THE TOTAL MASS OF THE ALLOY. 